1. Field of the Invention
This invention relates to fluidised beds, and more particularly to a method of operating a fluidised bed and to fluidised bed apparatus.
2. Description of the Prior Art
In U.S. patent application Ser. No. 717,171, filed Aug. 24, 1976, now abandoned the disclosure of which is incorporated herein by reference, there is described a method of thermally treating glass in which the hot glass is immersed in a fluidised bed of particulate material. Fluidisation of the particulate material is effected within a tank by passing fluidising gas through a porous membrane forming the base of the tank. Prior to immersion of the glass the fluidised bed is in a quiescent uniformly expanded state of particulate fluidisation.
The fluidised bed of particulate material in a quiescent uniformly expanded state of particulate fluidisation can be defined in terms of gas flow through the bed and the expanded height of the bed. the quiescent uniformly expanded state of particulate fluidisation has been found to exist between a lower limit of gas velocity at incipient fluidisation, that is the velocity at which the particles just become suspended in the uniformly distributed upwardly flowing gas, and an upper limit of gas velocity at which maximum expansion of the bed occurs and the top surface of the bed is tranquil and undisturbed by blubbing. A gas velocity higher than that which effects maximum expansion of the bed results in the development of extensive bubbling in the bed and at the onset of such bubbling there may be a partial reduction of the bed height.
The invention of the above-mentioned Patent Application is particularly applicable for the thermal toughening of flat or bent glass sheets, such as those used singly as motor vehicle windscreens, sidelights or backlights, or as part of a laminated motor vehicle windscreen, or for use in construction of windscreen assemblies for aircraft and railway locomotives.
In United Kingdom patent specification No. 774,305, thermal toughening of a glass sheet was proposed by a method in which a hot glass sheet is immersed in a freely bubbling bed of particulate material but such a process has not been brought into commercial use hitherto.
The problem which we have found when attempting to operate such a freely bubbling fluidised bed for the thermal toughening of glass sheets is the high incidence of fracture of the glass sheets which occurs during their treatment in the fluidised bed. A freely bubbling bed has also been found to distort the shape of the glass sheets due to the irregular forces to which the glass sheets are subjected in a freely bubbling bed.
By using a fluidised bed of particulate material which is in a quiescent uniformly expanded state of particulate fluidisation a successful commercial yield of whole glass sheets is achieved, there being very few fractures of the glass sheets while the toughening stresses are being developed in the glass sheets. It has also been found that the use of such a fluidised bed has very little effect on the shape of the glass sheets.
For maintaining stable operation of a fluidised bed in the quiescent uniformly expanded state of particulate fluidisation there is a narrow range of fluidising gas velocities, between the lower limit of gas velocity at incipient fluidisation and the upper limit of gas velocity at the bed has a maximum expansion. For gas velocities above the upper limit there is general bubbling of the bed. Within the velocity limits for fluidisation in a quiescent uniformly expanded state of particulate fluidisation it has been found difficult to avoid the occurrence of localised bubbling in the bed, which in some cases can engender general bubbling in the bed. Another form of instability which arises is that of the formation of irregular currents of material in the bed. Both these effects are difficult to suppress once started. Such instabilities are particularly prone to arise in deep, e.g. 1 meter deep, fluidised beds such as are required for the processing of large glass sheets, for example of suitable size of motor vehicle windscreens.
In a paper entitled "The Influence of the Gas Distributor on Bed Expansion, Bubble Size, and Bubble Frequency in Fluidised Beds" by D. Geldart, and J. R. Kelsey, I. Chem. E. Symposium Series No. 30 (1968) pages 114 to 125, there is a study of the types of instability occuring in bubbling fluidised beds of sand. The pressure drop across the distributor through which fluidising gas was supplied, and the geometry of that distributor was studied in terms of the stability of bubbling in a bed up to 1.5 meters deep. The distributors employed included a perforated plate with underlying paper layers, a combination of a perforated plate with a porous plate, and a porous plate alone. The ratio of the pressure drop across the distributor to the pressure drop across the bed for minimum fluidisation was in the range 0.017 to 9.9 for a cylindrical bed and in the range 0.0056 to 1.19 for a "two-dimensional" bed of rectangular cross section. For bubble-phase fluidisation observations were made with a range of values of that ratio from 0.08 to 23.0 for a cylindrical bed and from 0.023 to 5.35 for a "two-dimensional" bed. The authors concluded that at low values of the ratio of pressure drop across the distributor to pressure drop across the bed, there was instability of bubbling, and that increasing the ratio from 0.1 to 10, measured at minimum fluidisation, had no observable effect on the behaviour of the bed. With the distributors and pressure ratios described only bubbling fluidisation could be achieved and such beds are unsuitable for the thermal processing of large glass sheets.
In a book entitled "Fundamental Aspects of Fluidised Bed Coating" by Muharrem Elmas, Deletsche Hitgevers Maatschappii N.V., Delft, 1969, there is described the use of a shallow bed of polyethylene particles about 5 cm deep fluidised to a state of homogeneous fluidisation produced by varying the gas flow rate to a value in the range 1.0 to 1.4 producing minimum fluidisation. The pressure drop across the porous bronze plate used was at least equal to the weight of the bed. Hot objects of metal, glass ceramics or plastics were dipped into the bed in order to coat the objects. Such a bed has the disadvantage that the gas flow rate and pressure drop across the porous plate are inadequate to produce and maintain homogeneous fluidisation in a deep bed of material suitable for the thermal treatment of glass, in particular large sheets of glass for vehicle windscreens, sidelights or backlights.
It has also been proposed in United Kingdom Patent Specification No. 709,265 to employ rolled wire filter cloth as a gas-pervious support for a fluidised bed, with the pressure drop across the gas-pervious support of the same order as the pressure drop across the fluidised bed.
The present invention is based on the discovery that stable operation of a fluidised bed in a quiescent uniformly expanded state of particulate fluidisation can be obtained by appropriate choice of membrane to create a high pressure drop across the membrane due to the flow of fluidising gas through the porous membrane through which the fluidising gas enters the bed.
It is a main object of the invention to employ this discovery for improving stability of maintenance of a fluidised bed in a quiescent uniformly expanded state of particulate fluidisation for thermally treating glass articles.
It is another object of the invention to promote uniform gaseous flow into the particulate material to be maintained as a deep homogeneous bed in said quiescent uniformly expanded state of particulate fluidisation.